Communication system, relay device, communication terminal, and base station

ABSTRACT

Provided is a communication system including a plurality of base stations, a plurality of communication terminals that communicates with one of the plurality of base stations, and a relay device, the relay device including a selection unit that selects a communication terminal to be relayed from among the plurality of communication terminals on the basis of communication quality information received from each of the plurality of communication terminals, and a relay unit that relays communication between the communication terminal selected by the selection unit and the corresponding base station.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.13/496,499, filed Mar. 16, 2012, which is a National Stage ofPCT/JP2010/063808, filed Aug. 16, 2010, which claims the benefit ofpriority from Japanese Patent Application No. 2009-220483, filed Sep.25, 2009, the contents of which is incorporated in its entirety.

TECHNICAL FIELD

The present invention relates to a communication system, a relay device,a communication terminal, and a base station.

BACKGROUND ART

In IEEE (Institute of Electrical and Electronics Engineers) 802.16j, arelay technology is standardized. In addition, in 3GPP (Third GenerationPartnership Project) LTE-A (Long Term Evolution Advanced), a technologyof using a relay device (relay station) is also actively studied inorder to realize an improvement in the throughput of a communicationterminal located at a cell edge.

Such a relay device, upon receiving a signal transmitted from a basestation in a downlink, amplifies the signal and transmits the amplifiedsignal to a communication terminal. By performing such relay, the relaydevice can increase the signal-to-noise ratio compared to when a signalis transmitted directly from the base station to the communicationterminal. Likewise, in an uplink, the relay device can also maintain thehigh signal-to-noise ratio by relaying a signal transmitted from thecommunication terminal to the base station. Such a relay device isdescribed in, for example, Non-Patent Literature 1 to 3.

CITATION LIST Non-Patent Literature

-   Non-Patent Literature 1: R1-090015, “Consideration on Relay.ppt”,    China Potevio, CATT, January 2009-   Non-Patent Literature 2: R1-090065, “Joint analog network coding and    Relay”, Alcatel-Lucent, January 2009-   Non-Patent Literature 3: R1-091803, “Understanding on Type 1 and    Type 2 Relay”, Huawei, May 2009

SUMMARY OF INVENTION Technical Problem

However, there has been no report about, when a plurality ofcommunication terminals exists in the relayable range of a relay device,how to relay the communication of which communication terminal.Therefore, a case is supposed in which communication of a communicationterminal, which should be relayed with a high degree of necessity, maynot be relayed but a communication terminal, which should be relayedwith a low degree of necessity, may be relayed.

The present invention has been made in view of the foregoing problems,and it is an object of the present invention to provide a communicationsystem, a relay device, a communication terminal, and a base stationthat are novel and improved, and are capable of selecting acommunication terminal to be relayed.

Solution to Problem

In order to solve the aforementioned problem, according to one aspect ofthe present invention, there is provided a communication systemincluding a plurality of base stations, a plurality of communicationterminals that communicates with one of the plurality of base stations,and a relay device, the relay device including a selection unit thatselects a communication terminal to be relayed from among the pluralityof communication terminals on the basis of communication qualityinformation received from each of the plurality of communicationterminals, and a relay unit that relays communication between thecommunication terminal selected by the selection unit and thecorresponding base station.

The relay device may further include a power setting unit that setstransmission power of a relay signal for the communication terminal tobe relayed so that a difference between the transmission power and apropagation loss of the relay signal between another communicationterminal and the relay device is below a predetermined value.

The relay device may further include a distance estimation unit thatestimates a distance between the relay device and the othercommunication terminal on the basis of a propagation loss of a referencesignal received from the other communication terminal, the referencesignal having known transmission power, and the power setting unit mayestimate a propagation loss of the relay signal between the othercommunication terminal and the relay device on the basis of the distanceestimated by the distance estimation unit.

The selection unit may preferentially select a communication terminalwith bad communication quality from among the plurality of communicationterminals.

The relay unit may transmit the relay signal for the communicationterminal to be relayed through beam forming.

The relay device may further include a power setting unit that setstransmission power of a relay signal for a base station corresponding tothe communication terminal to be relayed so that a difference betweenthe transmission power and a propagation loss of the relay signalbetween another base station and the relay device is below apredetermined value.

The relay device may further include a distance estimation unit thatestimates a distance between the relay device and the other base stationon the basis of a propagation loss of a reference signal received fromthe other base station, the reference signal having known transmissionpower, and the power setting unit may estimate a propagation loss of therelay signal between the other base station and the relay device on thebasis of the distance estimated by the distance estimation unit.

In order to solve the aforementioned problem, according to anotheraspect of the present invention, there is provided a relay deviceincluding a selection unit that selects a communication terminal to berelayed from among a plurality of communication terminals on the basisof communication quality information received from each of the pluralityof communication terminals that communicates with one of a plurality ofbase stations, and a relay unit that relays communication between thecommunication terminal selected by the selection unit and thecorresponding base station.

In order to solve the aforementioned problem, according to still anotheraspect of the present invention, there is provided a communicationterminal, wherein when the communication terminal is selected as acommunication terminal to be relayed by a relay device that selects acommunication terminal to be relayed from among a plurality ofcommunication terminals on the basis of communication qualityinformation received from each of the plurality of communicationterminals including the communication terminal that communicates withone of a plurality of base stations, the communication terminalcommunicates with the base station via the relay device.

In order to solve the aforementioned problem, according to yet anotheraspect of the present invention, there is provided a base station,wherein when a communication terminal that communicates with the basestation is selected as a communication terminal to be relayed by a relaydevice that selects a communication terminal to be relayed from among aplurality of communication terminals on the basis of communicationquality information received from each of the plurality of communicationterminals that communicates with one of a plurality of base stationsincluding the base station, the base station communicates with thecommunication terminal via the relay device.

Advantageous Effects of Invention

As described above, according to the present invention, a communicationterminal to be relayed can be adequately selected.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram showing the configuration of acommunication system in accordance with an embodiment of the presentinvention.

FIG. 2 is an explanatory diagram showing exemplary resource allocationwhen the same frequency is used in an UL and a DL.

FIG. 3 is an explanatory diagram showing exemplary resource allocationwhen different frequencies are used in an UL and a DL.

FIG. 4 is an explanatory diagram showing an exemplary format of a DLradio frame.

FIG. 5 is an explanatory diagram showing an exemplary format of an ULradio frame.

FIG. 6 is an explanatory diagram showing a connection process sequence.

FIG. 7 is an explanatory diagram showing a specific example of a MBSFNtransmission/reception process.

FIG. 8 is an explanatory diagram showing exemplary frequency allocationto each cell.

FIG. 9 is an explanatory diagram showing an interference model of a DLbeing focused in the present embodiment.

FIG. 10 is an explanatory diagram showing an interference model of an ULbeing focused in the present embodiment.

FIG. 11 is a functional diagram showing the configuration of acommunication terminal.

FIG. 12 is a functional diagram showing the configuration of a relaydevice.

FIG. 13 is a sequence diagram showing a flow in which the relay devicerelays the DL communication.

FIG. 14 is a sequence diagram showing a flow in which the relay devicerelays the UL communication.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the drawings, structural elements that havesubstantially the same function and structure are denoted by the samereference signs, and repeated explanation is omitted.

In addition, in this specification and the drawings, a plurality ofstructural elements that have substantially the same function andstructure and are denoted by the same reference signs may be followed bydifferent alphabets for distinction purposes. For example, a pluralityof structures that have substantially the same function and structureare distinguished as communication terminals 20A, 20B, and 20C asneeded. However, when there is no need to particularly distinguishbetween each of the plurality of structural elements that havesubstantially the same function and structure, only reference signs areassigned. For example, when there is no need to particularly distinguishbetween the communication terminals 20A, 20B, and 20C, they are simplyreferred to as communication terminals 20.

The “Description of Embodiments” will be described in accordance withthe following item order.

1. Basic Configuration of the Communication System

-   -   (Exemplary Resource Allocation to Each Link)    -   (Exemplary Format of Radio Frame)    -   (Connection Process Sequence)    -   (MBSFN)    -   (Exemplary Frequency Allocation to Each Cell)

2. Specific Configuration of the Communication System

-   -   (Interference Model being Focused)    -   (Configuration of the Communication Terminal)    -   (Configuration of the Relay Device)

3. Operation of Communication System

4. Conclusion

1. BASIC CONFIGURATION OF THE COMMUNICATION SYSTEM

First, the basic configuration of a communication system 1 in accordancewith an embodiment of the present invention will be described withreference to FIGS. 1 to 8. FIG. 1 is an explanatory diagram showing theconfiguration of the communication system 1 in accordance with anembodiment of the present invention. As shown in FIG. 1, thecommunication system 1 in accordance with an embodiment of the presentinvention includes base stations 10A and 10B, a backbone network 12,communication terminals 20A and 20B, and 20X, and relay devices 30A and30B.

The base station 10 manages the communication between the relay device30 and the communication terminal 20 existing in a cell that is formedby the base station 10. For example, the base station 10A managesscheduling information for communicating with the communication terminal20X existing in the cell, and communicates with the communicationterminal 20X in accordance with the scheduling information. In addition,the base station 10A also manages scheduling information forcommunicating with the relay device 30A existing in the cell andscheduling information for the relay device 30A and the communicationterminal 20A to communicate with each other.

Note that the management of the scheduling information can be performedby the joint cooperation of the base station 10 and the relay device 30,by the joint cooperation of the base station 10, the relay device 30,and the communication terminal 20, or by the relay device 30.

The relay device 30 relays the communication between the base station 10and the communication terminal 20 in accordance with the schedulinginformation managed by the base station 10. Specifically, the relaydevice 30, upon receiving a signal transmitted from the base station 10in a downlink, transmits a signal obtained by amplifying the signal tothe communication terminal 20 using the frequency/time in accordancewith the scheduling information. By performing such relay, the relaydevice 30 can increase the signal-to-noise ratio compared to when asignal is transmitted directly from the base station 10 to thecommunication terminal 20 located near a cell edge.

Likewise, in an uplink, the relay device 30 can also maintain the highsignal-to-noise ratio by relaying a signal transmitted from thecommunication terminal 20 to the base station 10 in accordance with thescheduling information managed by the base station 10. Although FIG. 1shows an example in which only the relay device 30A exists in the cellformed by the base station 10A, a plurality of relay devices 30 canexist in the cell formed by the base station 10A.

As the types of such relay device 30, Type 1 and Type 2 have beenproposed. The relay device 30 of Type 1 has an individual cell ID and ispermitted to operate its own cell. Thus, the relay device 30 of Type 1operates in such a way that it is recognized as the base station 10 bythe communication terminal 20. However, the relay device 30 of Type 1operates not entirely autonomously, and performs relay communicationwithin the range of resources that are allocated by the base station 10.

Meanwhile, the relay device 30 of Type 2 does not have an individualcell ID unlike Type 1, and assists in the direct communication betweenthe base station 10 and the communication terminal 20. For example,relay transmission technologies using Cooperative relay and Networkcoding have been studied. The characteristics of Type 1 and Type 2 thatare currently studied are shown in Table I below.

TABLE 1 Item Type 1 Type 2 Decision R1-091098 R1-091632 Type of Relay L2and L3 Relay L2 PHY Cell ID Own cell ID No cell ID Transparency Nontransparent Relay node Transparent Relay to UE node to UE New cellCreate new cell (another Not create new cell eNB) RF parametersOptimized parameters N/A HO Inter cell HO (generic HO) HO transparentlyto UE Control Generate synch. channel, Not generate its Channel RS,H-ARQ channel and own channel Generation scheduling information etc. butdecodes/forwards donor eNB's signal to UE Backward Support (appear as aRel-8 Support (able to relay also compatibility eNB to Rel-8 UE) to/fromRel-8 UE) LTE-A Support (it appear — (Forward differently than Rel-8 eNBcompatibility) to LTE-A UE) Awareness - (>Rel-8 eNB to LTE-A — to MS UEsor Relay) Cooperation Inter cell cooperation Intra cell cooperationBackhaul Higher Lower utilization Usage model Coverage extensionThroughput enhancement and coverage extension Cost Higher Lower

As described above, the communication terminal 20 communicates with thebase station 10 either directly or via the relay device 30 in accordancewith the scheduling information managed by the base station 10. Notethat examples of data that are transmitted/received by the communicationterminal 20 include voice data; music data such as music, lectures, orradio programs; still image data such as photographs, documents,paintings, or charts; and moving image data such as movies, televisionprograms, video programs, or game images. The communication terminal 20can be an information processing device having a wireless communicationfunction such as a portable phone or a PC (Personal computer).

The management server 16 is connected to each base station 10 via thebackbone network 12. The management server 16 has a function of an MME(Mobile Management Entity). In addition, the management server 16 canalso have a function of a serving gateway. Further, the managementserver 16 receives from each base station 10 management informationindicating the state of a cell formed by each base station 10, andcontrols communication in the cell formed by each base station 10 on thebasis of the management information. Note that the function of themanagement server 16 can be implemented with a plurality of physicallyseparated configurations.

(Exemplary Resource Allocation to Each Link)

Herein, resource allocation to each link will be described. Note that,hereinafter, the communication channel between the base station 10 andthe relay device 30 will be referred to as a relay link, thecommunication channel between the relay device 30 and the communicationterminal 20 will be referred to as an access link, and the directcommunication channel between the base station 10 and the communicationterminal 20 will be referred to as a direct link. In addition, thecommunication channel toward the base station 10 will be referred to asan UL (uplink), and the communication channel toward the communicationterminal 20 will be referred to as a DL (downlink). Note also thatcommunication through each link is performed on the basis of OFDMA.

The relay device 30, in order to prevent mutual interference between therelay link and the access link, separates the relay link and the accesslink from each other on the basis of the frequency or time. For example,the relay device 30 can separate the relay link and the access link inthe same direction from each other on the basis of TDD (Time DivisionDuplexing) using a common frequency.

FIG. 2 is an explanatory diagram showing exemplary resource allocationwhen the same frequency is used in the UL and the DL. As shown in FIG.2, a radio frame includes a sub-frame 0 to a sub-frame 9. In the exampleshown in FIG. 2, the relay device 30, in accordance with an instructionfrom the base station 10, recognizes the sub-frames 8 and 9 as theresources for the DL of the access link, and relays a signal transmittedfrom the base station 10 to the communication terminal 20 using thesub-frames 8 and 9.

Note that a PSC (Primary Synchronization Channel) and a SSC (SecondarySynchronization Channel) that are synchronization signals for thedownlink, and a PBCH (Physical Broadcast CHannel) are allocated to thesub-frames 0 and 5. In addition, paging channels are assigned to thesub-frames 1 and 6.

FIG. 3 is an explanatory diagram showing exemplary resource allocationwhen different frequencies are used in the UL and the DL. As shown inFIG. 3, a frequency f0 is used for the DL and a frequency f1 is used forthe UL. In the example shown in FIG. 3, the relay device 30, inaccordance with an instruction from the base station 10, recognizessub-frames 6 to 8 of the frequency f0 as the resources for the DL of theaccess link, and relays a signal transmitted from the base station 10 tothe communication terminal 20 using the sub-frames 6 to 8 of thefrequency f0.

Note that a PSC and an SSC that are synchronization signals for thedownlink are assigned to the sub-frames 0 and 5 of the frequency f0 (forthe DL), and paging channels are assigned to the sub-frame 4 and thesub-frame 9.

(Exemplary Format of Radio Frame)

Next, a specific exemplary frame format of each of a DL radio frame andan UL radio frame will be described with reference to FIG. 4 and FIG. 5.

FIG. 4 is an explanatory diagram showing an exemplary format of a DLradio frame. The DL radio frame includes sub-frames 0 to 9, and eachsub-frame includes two 0.5 ms slots. Each 0.5 ms slot includes sevenOFDM (Orthogonal Frequency Division Multiplexing) symbols.

As shown in FIG. 4, in the head 1 to 3 OFDM symbols of each sub-frame,control channels such as PCFICH (Physical Control Format IndicatorCHannel), PHICH (Physical Hybrid ARQ Indicator CHannel), and PDCCH(Physical Control CHannel) and arranged.

Note that each of the aforementioned channels includes the followinginformation as an example.

PCFICH: the number of symbols of PDCCH related to Layer 1 and Layer 2

PHICH: ACK/NACK in response to PUSCH

PDCCH: downlink control information, scheduling information forPDSCH/PUSC (the format of a modulation method, encoding ratio, or thelike)

In addition, one resource block (1 RB), which is the minimum unit ofresource allocation, includes six or seven OFDM symbols and 12sub-carriers as shown in FIG. 4. A demodulation reference (a referencesignal) is arranged in part of the resource block.

Further, SSC, PBCH, and PSC are arranged in the sub-frames 0 and 5.Furthermore, a free portion in the radio frame shown in FIG. 4 is usedas a PDSCH (Physical Downlink Shared CHannel).

FIG. 5 is an explanatory diagram showing an exemplary format of the ULradio frame. Like the DL radio frame, the UL radio frame includessub-frames 0 to 9, and each sub-frame includes two 0.5 ms slots. Each0.5 ms slot includes seven OFDM symbols.

As shown in FIG. 5, a demodulation reference (a reference signal) isarranged in each of the 0.5 ms slots, and CQI measurement references arearranged in a dispersed manner. The base station 10 or the relay device30 on the receiving side performs channel estimation using thedemodulation reference, and demodulates a received signal in accordancewith the channel estimation result. In addition, the base station 10 orthe relay device 30 on the receiving side acquires CQI between the basestation 10 or the relay device 30 and the relay device 30 or thecommunication terminal 20 on the transmitting side by measuring the CQImeasurement reference.

Further, a free portion in the radio frame shown in FIG. 5 is used as aPUSCH (Physical Uplink Shared CHannel). Note that, when a CQI report isrequested, the communication terminal 20 or the relay device 30transmits the CQI report using the PUSCH.

(Connection Process Sequence)

Next, a process sequence for connecting the relay device 30 or thecommunication terminal 20 and the base station 10 will be described withreference to FIG. 6.

FIG. 6 is an explanatory diagram showing a connection process sequence.First, as shown in FIG. 6, the relay device 30 or the communicationterminal 20 transmits an RACH (Random Access CHannel) preamble to thebase station 10 (S62). The base station 10, upon receiving the RACHpreamble, acquires TA (Timing Advance) information, and transmits the TAinformation together with allocation resource information to the relaydevice 30 or the communication terminal 20 (S64). If the base station 10is able to grasp the transmission timing of the RACH preamble, forexample, the base station 10 can acquire as the TA information thedifference between the transmission timing and the reception timing ofthe RACH preamble.

After that, the relay device 30 or the communication terminal 20transmits an RRC connection request to the base station 10 usingresources indicated by the allocation resource information (S66). Thebase station 10, upon receiving the RRC connection request, transmits anRRC connection resolution indicating the source of transmission of theRRC connection request (S68). Accordingly, the relay device 30 or thecommunication terminal 20 is able to check if the base station 10 hasreceived the RRC connection request or not.

Next, the base station 10 transmits to the management server 16, whichhas a function of an MME, a connection request indicating that the relaydevice 30 or the communication terminal 20 is requesting a service(S70). The management server 16, upon receiving the connection request,transmits information for performing setup on the relay device 30 or thecommunication terminal 20 through connection

Then, the base station 10 transmits RRC connection setup to the relaydevice 30 or the communication terminal 20 on the basis of theconnection setup from the management server 16 (S74), whereupon therelay device 30 or the communication terminal 20 performs a connectionsetup. After that, the relay device 30 or the communication terminal 20transmits to the base station 10 RRC connection complete indicating thatthe connection setup is complete (S76).

Accordingly, the connection between the relay device 30 or thecommunication terminal 20 and the base station 10 is completed, wherebythey become able to communicate with each other. Note that theaforementioned connection process sequence is only exemplary, and therelay device 30 or the communication terminal 20 and the base station 10can be connected through another sequence.

(MBSFN)

Next, MBSFN (Multi-media Broadcasting Single Frequency Network)transmission performed by the base station 10, and an exemplaryoperation of the relay device 30 performed in response to the MBSFNtransmission will be described.

MBSFN is a mode in which a plurality of base stations 10 concurrentlyperforms data broadcast transmission using the same frequency. Thus,according to MBSFN, the relay device 30 of Type 1, which virtuallyoperates as a base station, transmits a control channel and the like forthe DL using the same frequency as that of the base station 10.Hereinafter, a specific flow of the MBSFN transmission/reception processwill be described with reference to FIG. 7.

FIG. 7 is an explanatory diagram showing a specific example of the MBSFNtransmission/reception process. First, as shown in FIG. 7, the basestation 10 and the relay device 30 concurrently transmit PDCCH. Herein,following the PDCCH, the base station 10 transmits R-PDCCH forcontrolling the relay in addition to the PDSCH for the communicationterminal 20. After the R-PDCCH, PDSCH (data to be relayed) for the relaydevice 30 is transmitted. Note that a non-transmission section isprovided after the PDSCH for the relay device 30.

The relay device 30, after transmitting the PDCCH, undergoes a sectionof switching to a reception process, and receives the PDSCH (data to berelayed) from the base station 10. Then, the relay device 30 switchesthe reception process to a transmission process in the non-transmissionsection provided after the PDSCH (data to be relayed) from the basestation 10. Further, the relay device 30 adds PDCCH to the decoded PDSCH(data to be relayed) in the next step, and relay-transmits it to thecommunication terminal 20.

Accordingly, existing communication terminals, which are not based onthe presence of the relay device 30, can relish the relay by the relaydevice 30 without confusion.

(Exemplary Frequency Allocation to Each Cell)

Next, exemplary frequency allocation to each cell when a plurality ofcells is adjacent to one another will be described.

FIG. 8 is an explanatory diagram showing exemplary frequency allocationto each cell. When each cell includes three sectors, allocatingfrequencies f1 to f3 to the respective sectors as shown in FIG. 8 allowsinterference of the frequencies at the cell boundary to be suppressed.Such allocation is particularly effective in a densely populated areawith high traffic.

Note that in LTE-A, in order to realize high end-to-end throughput, avariety of new technologies have been studied such as spectrumaggregation, network MIMO, uplink multiuser MIMO, and relaytechnologies. Therefore, with the advent of new mobile applications withhigh throughput, there is a possibility that frequency resources maybecome depleted even in suburban areas. Further, in the introduction ofLTE-A, there is a possibility that introduction of the relay device 30may become activated in order to realize low-cost infrastructuredevelopment.

2. SPECIFIC CONFIGURATION OF THE COMMUNICATION SYSTEM

The basic configuration of the communication system 1 in accordance withthe present embodiment has been described above with reference to FIG. 1to FIG. 8. Next, the specific configuration of the communication system1 in accordance with the present embodiment will be described withreference to FIGS. 9 to 12.

(Interference Model being Focused)

FIG. 9 is an explanatory diagram showing an interference model of a DLbeing focused in the present embodiment. In the present embodiment, asshown in FIG. 9, a case is considered in which, a relay device 30A islocated at a position where the relay device 30A is able to receivePDCCH from a plurality of base stations 10 (base stations 10A and 10B),and is located at a position where the relay device 30A is able toreceive signals from communication terminals 20 (communication terminals20A and 20B) belonging the respective base stations 10.

In this case, the relay device 30A can relay both the communicationbetween the base station 10A and the communication terminal 20A and thecommunication between the base station 10B and the communicationterminal 20B. Herein, if the relay device 30A relays a signaltransmitted from the base station 10A to the communication terminal 20Awithout exercising any ingenuity, it is concerned that the signaltransmitted by the relay and a signal transmitted from the base station10B may interfere with each other at the communication terminal 20B.

FIG. 10 is an explanatory diagram showing an interference model of an ULbeing focused in the present embodiment. In FIG. 10 a relay device 30Ais located at a position where the relay device 30A is able to receivePDCCH from a plurality of base stations 10 (base stations 10A and 10B),and is located at a position where the relay device 30A is able toreceive signals from communication terminals 20 (communication terminals20A and 20B) belonging the respective base stations 10, as in FIG. 9.

In such a case, the relay device 30A can relay both the communicationbetween the base station 10A and the communication terminal 20A and thecommunication between the base station 10B and the communicationterminal 20B. Herein, if the relay device 30A relays a signaltransmitted from the communication terminal 20A to the base station 10Awithout exercising any ingenuity, it is concerned that the signaltransmitted by the relay and a signal transmitted from the communicationterminal 20B may interfere with each other at the base station 10B.

Further, a problem of, when a plurality of communication terminalsexists in the relayable range of a relay device, how to relay thecommunication of which communication terminal is yet to be solved.Therefore, a case is supposed in which communication of a communicationterminal, which should be relayed with a high degree of necessity, maynot be relayed but a communication terminal, which should be relayedwith a low degree of necessity, may be relayed.

The relay device 30 in accordance with the present embodiment has beenmade with the aforementioned background being focused. Thus, accordingto the relay device 30, it is possible to adequately selectcommunication to be relayed and suppress generation of interference dueto the relay. Hereinafter, the configuration of such relay device 30 inaccordance with the present embodiment will be described in conjunctionwith the configuration of the communication terminal 20.

(Configuration of the Communication Terminal)

FIG. 11 is a functional diagram showing the configuration of thecommunication terminal 20. As shown in FIG. 11, the communicationterminal 20 includes a plurality of antennae 220 a to 220 n, an analogprocessing unit 224, an AD/DA converter unit 228, and a digitalprocessing unit 230.

Each of the plurality of antennae 220 a to 220 n receives a radio signalfrom the base station 10 or the relay device 30 and acquires ahigh-frequency electrical signal, and then supplies the high-frequencysignal to the analog processing unit 224. In addition, each of theplurality of antennae 220 a to 220 n transmits a radio signal to thebase station 10 or the relay device 30 on the basis of a high-frequencysignal supplied from the analog processing unit 224. As thecommunication terminal 20 has a plurality of antennae 220 a to 220 n asdescribed above, it can perform MIMO (Multiple Input Multiple Output)communication or diversity communication.

The analog processing unit 224 converts a high-frequency signaltransmitted from the plurality of antennae 220 a to 220 n into abaseband signal by performing analog processing such as amplification,filtering, or down conversion. In addition, the analog processing unit224 converts a baseband signal supplied from the AD/DA converter unit228 into a high-frequency signal.

The AD/DA converter unit 228 converts the baseband signal in an analogformat supplied from the analog processing unit 224 into a digitalformat, and supplies it to the digital processing unit 230. In addition,the AD/DA converter unit 228 converts the baseband signal in a digitalformat supplied from the digital processing unit 230 into an analogformat, and supplies it to the analog processing unit 224.

The digital processing unit 230 includes a synchronizing unit 232, adecoder 234, an encoder 240, and a control unit 242. Among them, thesynchronizing unit 232, the decoder 234, the encoder 240, and the likefunction as a communication unit for communicating with the base station10 or the relay device 30, together with the plurality of antennae 220 ato 220 n, the analog processing unit 224, and the AD/DA converter unit228.

The synchronizing unit 232 is supplied with a synchronization signalsuch as a PSC or a SSC, which has been transmitted from the base station10 or the relay device 30, from the AD/DA converter unit 228, andperforms a synchronization process on a radio frame on the basis of thesynchronization signal. Specifically, the synchronizing unit 232computes the correlation between the synchronization signal and a knownsequence pattern, and detects the peak position of the correlation,thereby synchronizing a radio frame.

The decoder 234 decodes the baseband signal supplied from the AD/DAconverter unit 228 to obtain the received data. Note that the decodingcan include, for example, a MIMO reception process and an OFDMdemodulation process.

The encoder 240 encodes the data to be transmitted such as PUSCH, andsupplies it to the AD/DA converter unit 228. Note that the encoding caninclude, for example, a MIMO transmission process and an OFDM modulationprocess.

The control unit 242 controls the entire operation in the communicationterminal 20 such as a transmission process, a reception process, and aprocess of connecting to the relay device 30 or the base station 10. Forexample, the communication terminal 20, under the control of the controlunit 242, performs a transmission process and a reception process usingresource blocks allocated by the base station 10. Note that the controlunit 242 controls a transmission process in accordance with atransmission parameter specified by the base station 10 or the relaydevice 30. For example, when the base station 10 has specified a TPC(Transmit Power Control) parameter for the communication terminal 20using PDCCH, the control unit 242 controls a transmission process inaccordance with the TPC parameter specified by the base station 10.

Meanwhile, when the base station 10 or the relay device 30 has requesteda CQI report to the communication terminal 20 using PDCCH, the digitalprocessing unit 230 measures the channel quality (e.g., received power)using a demodulation reference transmitted from the base station 10 orthe relay device 30. The control unit 242 generates a CQI report on thebasis of the aforementioned measurement result, and supplies thegenerated CQI report to the encoder 240. Consequently, the CQI report istransmitted to the base station 10 or the relay device 30 using PUSCH.

(Configuration of the Relay Device)

Next, the configuration of the relay device 30 will be described withreference to FIG. 12.

FIG. 12 is a functional block diagram showing the configuration of therelay device 30. As shown in FIG. 12, the relay device 30 includes aplurality of antennae 320 a to 320 n, an analog processing unit 324, anAD/DA converter unit 328, and a digital processing unit 330.

Each of the plurality of antennae 320 a to 320 n receives a radio signalfrom the base station 10 or the communication terminal 20 and acquires ahigh-frequency electrical signal, and then supplies the high-frequencysignal to the analog processing unit 324. In addition, each of theplurality of antennae 320 a to 320 n transmits a radio signal to thebase station 10 or the communication terminal 20 on the basis of ahigh-frequency signal supplied from the analog processing unit 324. Asthe relay device 30 has a plurality of antennae 320 a to 320 n asdescribed above, it can perform MIMO communication or diversitycommunication.

The analog processing unit 324 converts a high-frequency signal suppliedfrom the plurality of antennae 320 a to 320 n into a baseband signal byperforming analog processing such as amplification, filtering, or downconversion. In addition, the analog processing unit 324 converts abaseband signal supplied from the AD/DA converter unit 328 into ahigh-frequency signal.

The AD/DA converter unit 328 converts the baseband signal in an analogformat supplied from the analog processing unit 324 into a digitalformat, and supplies it to the digital processing unit 330. In addition,the AD/DA converter unit 328 converts the baseband signal in a digitalformat supplied from the digital processing unit 330 into an analogformat, and supplies it to the analog processing unit 324.

The digital processing unit 330 includes a synchronizing unit 332, adecoder 334, a buffer 338, an encoder 340, a control unit 342, a relayselection unit 344, a distance estimation unit 346, and a power settingunit 348. Among them, the synchronizing unit 332, the decoder 334, theencoder 340, and the like function as a receiving unit, a transmittingunit, and a relay unit for communicating with the base station 10 or thecommunication terminal 20, together with the plurality of antennae 320 ato 320 n, the analog processing unit 324, and the AD/DA converter unit328.

The synchronizing unit 332 is supplied with a synchronization signal,which has been transmitted from the base station 10, from the AD/DAconverter unit 328, and performs a synchronization process on a radioframe on the basis of the synchronization signal. Specifically, thesynchronizing unit 332 computes the correlation between thesynchronization signal and a known sequence pattern, and detects thepeak position of the correlation, thereby synchronizing a radio frame.

The decoder 334 decodes the baseband signal supplied from the AD/DAconverter unit 328, and obtains relay data addressed to the base station10 or to the communication terminal 20. Note that the decoding caninclude, for example, a MIMO reception process, an OFDM demodulationprocess, and an error correction process.

The buffer 338 temporally stores the relay data addressed to the basestation 10 or to the communication terminal 20 obtained by the decoder334. Then, under the control of the control unit 342, the relay dataaddressed to the communication terminal 20 is read from the buffer 338into the encoder 340 using resource blocks for the DL of the accesslink. Likewise, under the control of the control unit 342, the relaydata addressed to the base station 10 is read from the buffer 338 intothe encoder 340 using resource block for the UL of the relay link.

The encoder 340 encodes the relay data supplied from the buffer 338, andsupplies it to the AD/DA converter unit 328. Note that the encoding caninclude, for example, a MIMO transmission process and OFDM modulationprocess.

(Relay Selection)

The relay selection unit 344, when the relay device 30 is located at aposition where the relay device 30 is able to relay a plurality ofcommunications, selects any of or all of the communications as thecommunication to be relayed. For example, the relay selection unit 344of the relay device 30A shown in FIG. 9 selects which of thecommunication between the base station 10A and the communicationterminal 20A and the communication between the base station 10B and thecommunication terminal 20B is to be relayed. Hereinafter, the criteriaof selection by the relay selection unit 344 will be specificallydescribed.

As the relay device 30A is able to receive PDCCH from both the basestations 10A and 10B, the relay selection unit 344 acquires schedulinginformation for an UL toward each base station 10 from the PDCCH. Inaddition, as the relay device 30A is able to receive PUSCH from both thecommunication terminals 20A and 20B, the relay selection unit 344acquires a CQI report from the PUSCH. Note that the relay selection unit344 can determine from which of the communication terminals 20 eachPUSCH has been transmitted on the basis of the scheduling informationfor the UL.

Then, the relay selection unit 344 selects the communication to berelayed on the basis of the acquired CQI report (communication qualityinformation). Herein, it is considered that relaying the communicationbetween the base station 10 and the communication terminal 20 would beof greater significance as the communication quality of the direct linkbetween the base station 10 and the communication terminal 20 is worse.Therefore, the relay selection unit 344 can preferentially select thecommunication of the direct link with worse communication quality ineach of the UL and the DL.

(Case of DL Communication)

For example, in the example shown in FIG. 9, when the communicationquality indicated by the CQI report transmitted from the communicationterminal 20A is worse than the communication quality indicated by theCQI report transmitted from the communication terminal 20B, the relayselection unit 344 can select the DL communication in the direction fromthe base station 10A to the communication terminal 20A. That is, therelay selection unit 344 can select the DL communication in thedirection from the base station 10A to the communication terminal 20A asthe target to be relayed when “CQI_level_communication terminal20A<CQI_(—) level_(—) terminal 20B.”

(Case of UL Communication)

Similarly, in the example shown in FIG. 10, when the communicationquality indicated by the CQI report transmitted from the communicationterminal 20A is worse than the communication quality indicated by theCQI report transmitted from the communication terminal 20B, the relayselection unit 344 can select the UL communication in the direction fromthe communication terminal 20A to the base station 10A. That is, therelay selection unit 344 can select the UL communication in thedirection from the communication terminal 20A to the base station 10A asthe target to be relayed when “CQI_level_communication terminal20A<CQI_level_ terminal 20B.”

Although the description has been made above of an example in which therelay selection unit 344 determines the communication with badcommunication quality on the basis of the CQI report, the presentembodiment is not limited thereto. For example, as a TPC parameterspecified by the base station 10 for the communication terminal 20changes in accordance with the state of the direct link between the basestation 10 and the communication terminal 20, the TPC parameter can alsobe recognized as an index indicating the communication quality of thedirect link. Thus, the relay selection unit 344 can preferentiallyselect communication, which is specifically performed with a high outputsignal, as the target to be relayed on the basis of the TPC parameterspecified through PDCCH by the base station 10 for the communicationterminal 20.

(Distance Estimation)

The distance estimation unit 346 estimates the distance from each basestation 10 and the distance from each communication terminal 20 locatedin the range in which communication is possible. For example, thedistance estimation unit 346 of the relay device 30A shown in FIG. 9estimates the distance from the base station 10A, the distance from thebase station 10B, the distance from the communication terminal 20A, andthe distance from the communication terminal 20B.

Specifically, the distance estimation unit 346 estimates the distance onthe basis of a propagation loss of a reference signal whose transmissionpower and phase are known, transmitted from each base station 10 andeach communication terminal 20. For example, the distance estimationunit 346 can calculate a propagation loss of a reference signal(demodulation reference) transmitted from the communication terminal 20Aand estimate the distance from the communication terminal 20A on thebasis of the calculated propagation loss. Similarly, the distanceestimation unit 346 can calculate a propagation loss of a referencesignal transmitted from the base station 10B and estimate the distancefrom the base station 10B on the basis of the calculated propagationloss.

(Transmission Power Setting)

The power setting unit 348 sets transmission power for performing therelay selected by the relay selection unit 344. Hereinafter,transmission power set by the power setting unit 348 in each of a casein which the target to be relayed is the DL communication and a case inwhich the target to be relayed is the UL communication will bedescribed.

(Case of DL Communication)

In the example shown in FIG. 9, when the relay selection unit 344 hasselected the DL communication in the direction from the base station 10Ato the communication terminal 20A as the target to be relayed, a signaltransmitted for relay from the relay device 30A to the communicationterminal 20A is received by the communication terminal 20B as noisecomponents. Further, if the noise components exceed the permissibleinterference level of the communication terminal 20B, there is apossibility that interference may be generated. Therefore, the powersetting unit 348 sets the transmission power of a signal for thecommunication terminal 20A so that interference would not be generatedat the communication terminal 20B. Specifically, the power setting unit348 can set the transmission power so that Qos expected by the basestation 10A/communication terminal 20A is satisfied and also Formula 1below is satisfied.[Math. 1]Transmission power[dB]<permissible interference level of thecommunication terminal 20B[dB]+propagation loss between the relay device30A and the communication terminal 20B[dB]  (Formula 1)

In Formula 1 above, the permissible interference level of thecommunication terminal 20B can be the SINR required at the minimum rateof the communication terminal 20B indicated by device authenticationinstitutions. Further, the power setting unit 348 can estimate apropagation loss between the relay device 30A and the communicationterminal 20B on the basis of the distance between the relay device 30Aand the communication terminal 20B estimated by the distance estimationunit 346. Note that the power setting unit 348 can set the minimumtransmission power within the range that Qos expected by the basestation 10A/communication terminal 20A is satisfied and also Formula 1above is satisfied, in view of reducing the power consumption.

When the transmission power that satisfies Formula 1 above is absent,the relay device 30 need not perform the relay. Alternatively, when therelay device 30 is authorized to schedule resources, it can reallocatethe resource blocks so that interference will not be generated.

(Case of UL Communication)

In the example shown in FIG. 10, when the relay selection unit 344 hasselected the UL communication in the direction from the communicationterminal 20A to the base station 10A as the target to be relayed, asignal transmitted for rely from the relay device 30A to the basestation 10A is received by the base station 10B as noise components.Further, if the noise components exceed the permissible interferencelevel of the base station 10B, there is a possibility that interferencemay be generated. Therefore, the power setting unit 348 sets thetransmission power of a signal for the base station 10A so thatinterference would not be generated at the base station 10B.Specifically, the power setting unit 348 can set the transmission powerso that Qos expected by the base station 10A/communication terminal 20Ais satisfied and also Formula 2 below is satisfied.[Math. 2]Transmission power[dB]<permissible interference level of the basestation 10B[dB]+propagation loss between the relay device 30A and thebase station 10B[dB]  (Formula2)

In Formula 2 above, the permissible interference level of the basestation 10B can be the SINR required at the minimum rate of the basestation 10B indicated by device authentication institutions. Further,the power setting unit 348 can estimate a propagation loss between therelay device 30A and the base station 10B on the basis of the distancebetween the relay device 30A and the base station 10B estimated by thedistance estimation unit 346. Note that the power setting unit 348 canset the minimum transmission power within the range that Qos expected bythe base station 10A/communication terminal 20A is satisfied and alsoFormula 2 above is satisfied, in view of reducing the power consumption.

When the transmission power that satisfies Formula 2 above is absent,the relay device 30 need not perform the relay. Alternatively, when therelay device 30 is authorized to schedule resources, it can reallocatethe resource blocks so that interference will not be generated.

(Control Unit)

The control unit 342 controls the transmission process so that a signalfor relay is transmitted to the base station 10 or the communicationterminal 20 selected by the relay selection unit 344 using thetransmission power set by the power setting unit 348. Further, thecontrol unit 342 can, in controlling the transmission process, control atransmission parameter such as an AMC (Advanced Modulation and Coding)parameter or a HARQ (Hybrid Automatic Repeat Request) parameter in amanner described below. Note that the control below can be performedeither alone or in combination.

(Case of DL Communication)

AMC

When the communication terminal 20, which is the relay destination, andthe relay device 30 have a positional relationship in which thereception level for a signal from the relay device 30 is sufficientlyhigher than the reception level at the communication terminal 20 fromthe direct link with high possibility and retransmission packets arerepeatedly transmitted through the direct link, the control unit 342 canperform overlay transmission of a relay signal using a Modulation-Codingparameter with a higher rate than that of the direct link. In such acase, the signal transmitted through the direct link is buried whenreceived by the communication terminal 20, but it is expected that therelay signal from the relay device 30 be decoded by the communicationterminal 20. Note that the relay device 30 can also transmit a relaysignal using a Modulation-Coding parameter with a higher rate than thatof the direct link, utilizing available time slots.

HARQ

When retransmission packets are repeatedly transmitted through thedirect link between the base station 10 and the communication terminal20, the control unit 342 can perform overlay transmission of a relaysignal using the same parameter as that of the retransmission packets.Note that the relay device 30 can also transmit a relay signal as a HARQpacket at a higher rate than that of the direct link utilizing availabletime slots.

Beam Forming

When the control unit 342 can estimate the relative direction of thecommunication terminal 20, which is the relay destination, the controlunit 342 can transmit a relay signal through beam forming. In such acase, the power setting unit 348 can set the transmission power on thebasis of the transmission power and propagation loss of a Null beam fora communication terminal 20 that is not the relay destination. Accordingto such beam forming, it becomes possible to select a plurality ofcommunication terminals 20 as the relay destinations and concurrentlytransmit relay signals to the plurality of communication terminals 20.

(Case of UL Communication)

AMC

When the base station 10, which is the relay destination, and the relaydevice 30 have a positional relationship in which the reception levelfor a signal from the relay device 30 is sufficiently higher than thereception level at the base station 10 from the direct link with highpossibility and retransmission packets are repeatedly transmittedthrough the direct link, the control unit 342 can perform overlaytransmission of a relay signal using a Modulation-Coding parameter witha higher rate than that of the direct link. In such a case, the signaltransmitted through the direct link is buried when received by the basestation 10, but it is expected that the relay signal from the relaydevice 30 be decoded by the base station 10. Note that the relay device30 can also transmit a relay signal using a Modulation-Coding parameterwith a higher rate than that of the direct link, utilizing availabletime slots.

HARQ

When retransmission packets are repeatedly transmitted through thedirect link between the base station 10 and the communication terminal20, the control unit 342 can perform overlay transmission of a relaysignal using the same parameter as that of the retransmission packets.Note that the relay device 30 can also transmit a relay signal as a HARQpacket at a higher rate than that of the direct link utilizing availabletime slots.

Beam Forming

When the control unit 342 can estimate the relative direction of thebase station 10, which is the relay destination, the control unit 342can transmit a relay signal through beam forming. In such a case, thepower setting unit 348 can set the transmission power on the basis ofthe transmission power and propagation loss of a Null beam for a basestation 10 that is not the relay destination. According to such beamforming, it becomes possible to select a plurality of base stations 10as the relay destinations and concurrently transmit relay signals to theplurality of base stations 10.

3. OPERATION OF COMMUNICATION SYSTEM

The specific configuration of the communication system 1 in accordancewith the present embodiment has been described above with reference toFIG. 9 to FIG. 12. Next, the operation of the communication system 1 inaccordance with the present embodiment will be described with referenceto FIG. 13 and FIG. 14. Note that the present embodiment is based on thefollowing points.

-   -   The relay device 30 uses a direct link, and has terminated the        procedures of up to “RRC connection complete” in accordance with        similar procedures to those of the communication terminal 20,        and has also determined the sub-cell ID, reference pattern        allocation, and the like.    -   The base station 10 and the relay device 30 belonging thereto        are synchronized.    -   Grouping information that indicates the relay device 30 and the        communication terminal 20 belonging to the relay device 30 is        given by the base station 10 in advance (the base station 10        determines the necessity of relay from a CQI report or TA        information, and allocates resources for relay if necessary).    -   Ptx_DL>>Ptx_RL and Ptx_DL>>Ptx_AL (Ptx: the maximum transmission        power), DL: direct link (direct link between the base station 10        and the communication terminal 20), AL: access link, and RL:        relay link    -   The primary object to be achieved is to take measures against        interference to the direct link, in particular, interference to        the direct link of the communication device (LTE UE) that is not        based on the presence of the relay device 30.

(Case of DL Communication)

FIG. 13 is a sequence diagram showing a flow in which the relay device30 relays the DL communication. As shown in FIG. 13, the relay device30, upon receiving PDCCH from the base station 10A (S404) and receivingPDCCH from the base station 10B (S408), acquires scheduling informationfrom each PDCCH (S412).

Next, the relay device 30, upon receiving a demodulation reference fromthe communication terminal 20A (S416) and receiving a demodulationreference from the communication terminal 20B (S420), estimates thedistance from the communication terminal 20A and the distance from thecommunication terminal 20B on the basis of a propagation loss of eachdemodulation reference (S424). Note that it is possible to determinefrom which communication terminal 20 each demodulation reference hasbeen transmitted on the basis of the scheduling information acquired inS412.

Further, when a CQI report is received from the communication terminal20A (S428) and a CQI report is received from the communication terminal20B (S432), the relay selection unit 344 selects which of thecommunication directed to the communication terminal 20A and thecommunication directed to the communication terminal 20B is to berelayed on the basis of the communication quality indicated by the CQIreport (S436). For example, the relay selection unit 344 canpreferentially select communication with bad communication quality.

After that, the power setting unit 348 sets the transmission power of asignal for the communication terminal 20 selected in S436 so that thereception level at the other communication terminal 20 becomes less thanor equal to the permissible interference level of the othercommunication terminal 20 (S440). Then, when the communication terminal20A is selected in S436, the relay device 30, upon receiving PDSCH fromthe base station 10A (S444), transmits the received PDSCH to thecommunication terminal 20A using the transmission power set by the powersetting unit 348 (S448). Note that the relay device 30 can transmit thePDSCH to the communication terminal 20A by adequately controlling aparameter such as AMC or HARQ.

(Case of UL Communication)

FIG. 14 is a sequence diagram showing a flow in which the relay device30 relays the UL communication. As shown in FIG. 14, the relay device30, upon receiving PDCCH from the base station 10A (S454) and receivingPDCCH from the base station 10B (S458), acquires scheduling informationfrom each PDCCH (S462).

Next, the relay device 30, upon receiving a reference signal from thebase station 10A (S466) and receiving a reference signal from the basestation 10B (S470), estimates the distance from the base station 10A andthe distance from the base station 10B on the basis of a propagationloss of each reference signal (S474).

Further, when a CQI report is received from the communication terminal20A (S478) and a CQI report is received from the communication terminal20B (S482), the relay selection unit 344 selects which of thecommunication directed to the base station 10A and the communicationdirected to the base station 10B is to be relayed on the basis of thecommunication quality indicated by the CQI report (S486). For example,the relay selection unit 344 can preferentially select communicationwith bad communication quality.

After that, the power setting unit 348 sets the transmission power of asignal for the base station 10 selected in S486 so that the receptionlevel at the other base station 10 becomes less than or equal to thepermissible interference level of the other base station 10 (S490).Then, when the base station 10A is selected in S486, the relay device30, upon receiving PUSCH from the communication terminal 20A (S494),transmits the received PUSCH to the base station 10A using thetransmission power set by the power setting unit 348 (S498). Note thatthe relay device 30 can transmit the PDSCH to the communication terminal20A by adequately controlling a parameter such as AMC or HARQ.

4. CONCLUSION

As described above, the relay device 30 in accordance with the presentembodiment can, when a plurality of base stations 10 and communicationterminals 20 exist in the range in which communication is possible,adequately select the communication to be relayed. Further, the relaydevice 30 in accordance with the present embodiment can transmit a relaysignal using transmission power that would not cause interference at abase station 10 or a communication terminal 20 that is not the relaydestination.

Although the preferred embodiments of the present invention have beendescribed in detail with reference to the appended drawings, the presentinvention is not limited thereto. It is obvious to those skilled in theart that various modifications or variations are possible insofar asthey are within the technical scope of the appended claims or theequivalents thereof. It should be understood that such modifications orvariations are also within the technical scope of the present invention.

For example, the steps in the process of the communication system 1 inthis specification need not necessarily be processed in a time-seriesorder in accordance with the order described in the sequence diagram.The steps in the process of the communication system 1 can be performedin an order different from that described in the sequence diagram, or beprocessed in parallel. For example, S404 and S408 in FIG. 13 can beconcurrently received by the relay device 30 or one of them can bereceived earlier. The same is true for S416 and S420 and S428 and S432.Further, for example, the same is also true for S454 and S458, S466 andS470, and S478 and S482 in FIG. 14.

It is also possible to create a computer program for causing built-inhardware in the relay device 30, such as a CPU, ROM, and RAM, to exert afunction that is equivalent to each of the aforementioned configurationsof the relay device 30. In addition, a storage medium having thecomputer program stored therein is also provided.

The invention claimed is:
 1. A communication system, comprising: a plurality of base stations; a plurality of communication terminals that each communicate with a corresponding base station of the plurality of base stations; and a relay device including circuitry configured to: select a communication terminal having a bad communication quality from the plurality of communication terminals based on communication quality information received from each of the plurality of communication terminals; set a transmission power for relaying communication between the selected communication terminal and the corresponding base station so that a difference between the transmission power and a propagation loss of relayed communication between another communication terminal and the circuitry satisfies a predetermined condition; and relay the communication between the selected communication terminal and the corresponding base station.
 2. The communication system according to claim 1, wherein the circuitry is further configured to: estimate a distance between the relay device and the selected communication terminal based on a propagation loss of a reference signal received by the circuitry from the selected communication terminal, the reference signal having a known transmission power; and estimate a propagation loss of the communication received by the circuitry from the selected communication terminal based on the estimated distance.
 3. The communication system according to claim 1, wherein satisfying the predetermined condition is when the difference is below a predetermined value.
 4. The communication system according to claim 1, wherein the circuitry is further configured to transmit the communication from the selected communication terminal to the corresponding base station through beam forming.
 5. A relay device, comprising: circuitry configured to select a communication terminal having a bad communication quality from a plurality of communication terminals based on communication quality information received from each of the plurality of communication terminals, each communication terminal of the plurality of communication terminals communicating with a corresponding base station of a plurality of base stations; set a transmission power for relaying communication between the selected communication terminal and the corresponding base station so that a difference between the transmission power and a propagation loss of relayed communication between another communication terminal and the circuitry satisfies a predetermined condition; and control at least one antenna of a plurality of the antennas to relay the communication between the selected communication terminal and the corresponding base station.
 6. The relay device according to claim 5, wherein the circuitry is further configured to: estimate a distance between the relay device and the selected communication terminal based on a propagation loss of a reference signal received by the circuitry from the selected communication terminal, the reference signal having known transmission power; and estimate a propagation loss of the communication received by the circuitry from the selected communication terminal based on the estimated distance.
 7. The relay device according to claim 5, wherein satisfying the predetermined condition is when the difference is below a predetermined value.
 8. The relay device according to claim 5, wherein the circuitry is further configured to control the at least one antenna to transmit the communication from the selected communication terminal to the corresponding base station through beam forming.
 9. A communication terminal, comprising: circuitry configured to control at least one antenna of a plurality of antennas to communicate, via a relay device, with a base station of a plurality of base stations when the communication terminal is selected by the relay device as a communication terminal having a bad communication quality from a plurality of communication terminals, wherein the relay device selects the communication terminal based on communication quality information received by the relay device from each of the plurality of communication terminals and sets a transmission power for relaying communication between the communication terminal and the base station so that a difference between the transmission power and a propagation loss of relayed communication between another communication terminal and the relay device satisfies a predetermined condition.
 10. The communication terminal according to claim 9, wherein the circuitry is further configured to control the at least one antenna to transmit a reference signal to the relay device, the reference signal having a known transmission power, a distance between the communication terminal and the relay device is estimated by the relay device on based a propagation loss of the reference signal, and a propagation loss of the relay signal is estimated by the relay device based on the estimated distance.
 11. The communication terminal according to claim 9, wherein satisfying the predetermined condition is when the difference is below a predetermined value.
 12. The communication terminal according to claim 9, wherein the relay device transmits the communication from the communication terminal to the base station through beam forming.
 13. A base station, comprising: circuitry configured to control an antenna to communicate, via a relay device, with a communication terminal when the communication terminal is selected by the relay device as a communication terminal having a bad communication quality from a plurality of communication terminals, wherein the relay device selects the communication terminal based on communication quality information received by the relay device from each of the plurality of communication terminals and sets a transmission power for relaying communication between the selected communication terminal and the circuitry so that a difference between the transmission power and a propagation loss of relayed communication between another communication terminal and the relay device satisfies a predetermined condition.
 14. The base station according to claim 13, wherein a distance between the relay device and the selected communication terminal is estimated by the relay device based on a propagation loss of a reference signal received by the relay device from the selected communication terminal, the reference signal having a known transmission power, and a propagation loss of the relay signal is estimated by the relay device based on the estimated distance.
 15. The base station according to claim 13, wherein satisfying the predetermined condition is when the difference is below a predetermined value.
 16. The base station according to claim 13, wherein the relay device is further configured to transmit the communication the selected communication terminal to the circuitry through beam forming.
 17. The communication system according to claim 1, wherein the circuitry is configured to select, based on the communication quality information received, the communication terminal having a lowest communication quality of the plurality of communication terminals.
 18. The relay device according to claim 5, wherein the circuitry is configured to select, based on the communication quality information received, the communication terminal having a lowest communication quality of the plurality of communication terminals.
 19. The communication terminal according to claim 9, wherein the communication terminal has a lower communication quality than the plurality of communication terminals.
 20. The base station according to claim 13, wherein the relay device selects, based on the communication quality information received, the communication terminal having a lowest communication quality of the plurality of communication terminals. 